Luke78
Gas Engineer
- Messages
- 28
Hello All-
(Usual caveats apply: I am the rather unusual part-time variety of a GSRE, training self-funded and slowly trying to make my way and gain experience- mainly do inspections and safety checks etc, have been on the Baxi fault-finding course relatively recently, don't profess to know it all, just looking for advice/opinions from anyone disposed to give them....)
All of that said:
Wondered if anyone can shed any light on my puzzlement...
Yesterday I serviced a pre-Greenstar Worcester 28i Junior Combi; GC no 47-311-70 customer thinks it is "at least" 10 years old.
I readily confess it is my first experience of this boiler and so I was mildly bemused to find that it has no diverter valve- A quick look inside the combustion chamber, combined with perusing the manual leads to me to deduce that it does not have one because the DHW flow and CH circuit both pass through the primary heat exchanger (there not, apparently, being a secondary!)
I did usual checks on burner pressure, gas rate, etc and all is within parameters, also vacuumed and brushed out burner rail, combustion chamber and even removed fan and cleaned the vanes out with vacuum set to "blow", reassembled, left the boiler merrily heating...
I was called back this afternoon by the customer who complained that hot water was only coming out luke warm (no pun intended!) and indeed it seems to be; Without any apparent pattern, water at the taps came out varying between 20 and 50 degrees.
There are no flashing lights etc to correspond with any error conditions stated in manual, front panel appears as in normal operation, boiler responds to demands from controls or taps in correct way (other than varying temperature at taps) and burner lights.
Customer also reports that the boiler was "making a noise as if running" which turned out to be the pump running continuously... Instructions say it should only run for 3 mins after demand stops.
Investigated inconsistent water temp first-
I would usually reason that DHW temperature dropping might be due to gas valve modulating, perhaps owing to feedback to the PCB from the DHW temp sensor-
Connected manometer to the burner pressure test point and ran boiler with just a DHW demand, and also while Central heating was operating, allowing to cool in between etc... Pressure is consistently 14.7 mb as per spec for max rate whenever taps are run, yet water temp at taps varies from demand to demand. Not sure if it is just my perception but I thought that maybe DHW came out hotter if the central heating had already been running.
I would usually reason that DHW temperature dropping might be due to gas valve modulating, perhaps owing to feedback to the PCB from the DHW temp sensor, but as stated, burner pressure remains constant so don't think this is the reason.
I am wondering if it is due to a tired/defective/ blocked heat exchanger, but would have expected that to possibly affect the DHW flow rate (i.e. pressure at taps to reduce) which doesn't seem to happen...
My own assessment (which of course, could be wrong!) is that, with the controls and sensors arranged as they are on the boiler, the only thing that could affect DHW temperature is the burner rate, there being no diverter valve to give priority to demands for DHW versus Central Heating...? I don't see that anything else could cause DHW temp to vary.
The next piece of the puzzle:
Working through steps in manual, I proceeded to check resistance recorded at temp sensor on DHW outlet from heat exchanger and whilst attempting to remove the first connector (and I wasn't rough with it!), the whole body of the sensor came away from the mounting disc, there apparently being a crack all round the bottom; not sure if this is just due to age, or whether it had been broken previously and stuck back on!
Checked the resistance anyway... Measured at approx 8500 ohms... Doesn't seem to vary if heated or cooled, but board seems happy to let boiler operate with sensor connected...
I would hazard a guess (as there are no resistance values given in the manual) that, if sensor is an NTC, then 8500 ohms would be toward the high end of the temperature range, and that perhaps, the PCB is seeing this and making the pump run to try and dissipate hot water from around the heat exchanger until temp sensor value changes and it thinks things have cooled down. However, as the sensor is defective and not registering the drop in temperature, the pump runs indefinitely?
And on the face of it, I don't see any reason, notwithstanding that the DHW temp sensor IS clearly defective, why this would cause inconsistent DHW temperature when the gas valve doesn't appear to be modulating.
Would welcome the wisdom and experience of those more learned than myself...
Finally... In case it's relevant... I suspect that the expansion vessel may have ruptured... Pressure was 0.6 bar, but when I depressed the schrader valve, a fair amount of water was discharged... as pressure was within spec and boiler seems to be operating satisfactorily, I opted not to drain it completely as I didn't want to create any more issues and leave customer with no heat at all.
Can't, on the face of it, see that this would have any effect on DHW temp, but perhaps it's just something I'm missing.
Any responses very gratefully received as always.
(Usual caveats apply: I am the rather unusual part-time variety of a GSRE, training self-funded and slowly trying to make my way and gain experience- mainly do inspections and safety checks etc, have been on the Baxi fault-finding course relatively recently, don't profess to know it all, just looking for advice/opinions from anyone disposed to give them....)
All of that said:
Wondered if anyone can shed any light on my puzzlement...
Yesterday I serviced a pre-Greenstar Worcester 28i Junior Combi; GC no 47-311-70 customer thinks it is "at least" 10 years old.
I readily confess it is my first experience of this boiler and so I was mildly bemused to find that it has no diverter valve- A quick look inside the combustion chamber, combined with perusing the manual leads to me to deduce that it does not have one because the DHW flow and CH circuit both pass through the primary heat exchanger (there not, apparently, being a secondary!)
I did usual checks on burner pressure, gas rate, etc and all is within parameters, also vacuumed and brushed out burner rail, combustion chamber and even removed fan and cleaned the vanes out with vacuum set to "blow", reassembled, left the boiler merrily heating...
I was called back this afternoon by the customer who complained that hot water was only coming out luke warm (no pun intended!) and indeed it seems to be; Without any apparent pattern, water at the taps came out varying between 20 and 50 degrees.
There are no flashing lights etc to correspond with any error conditions stated in manual, front panel appears as in normal operation, boiler responds to demands from controls or taps in correct way (other than varying temperature at taps) and burner lights.
Customer also reports that the boiler was "making a noise as if running" which turned out to be the pump running continuously... Instructions say it should only run for 3 mins after demand stops.
Investigated inconsistent water temp first-
I would usually reason that DHW temperature dropping might be due to gas valve modulating, perhaps owing to feedback to the PCB from the DHW temp sensor-
Connected manometer to the burner pressure test point and ran boiler with just a DHW demand, and also while Central heating was operating, allowing to cool in between etc... Pressure is consistently 14.7 mb as per spec for max rate whenever taps are run, yet water temp at taps varies from demand to demand. Not sure if it is just my perception but I thought that maybe DHW came out hotter if the central heating had already been running.
I would usually reason that DHW temperature dropping might be due to gas valve modulating, perhaps owing to feedback to the PCB from the DHW temp sensor, but as stated, burner pressure remains constant so don't think this is the reason.
I am wondering if it is due to a tired/defective/ blocked heat exchanger, but would have expected that to possibly affect the DHW flow rate (i.e. pressure at taps to reduce) which doesn't seem to happen...
My own assessment (which of course, could be wrong!) is that, with the controls and sensors arranged as they are on the boiler, the only thing that could affect DHW temperature is the burner rate, there being no diverter valve to give priority to demands for DHW versus Central Heating...? I don't see that anything else could cause DHW temp to vary.
The next piece of the puzzle:
Working through steps in manual, I proceeded to check resistance recorded at temp sensor on DHW outlet from heat exchanger and whilst attempting to remove the first connector (and I wasn't rough with it!), the whole body of the sensor came away from the mounting disc, there apparently being a crack all round the bottom; not sure if this is just due to age, or whether it had been broken previously and stuck back on!
Checked the resistance anyway... Measured at approx 8500 ohms... Doesn't seem to vary if heated or cooled, but board seems happy to let boiler operate with sensor connected...
I would hazard a guess (as there are no resistance values given in the manual) that, if sensor is an NTC, then 8500 ohms would be toward the high end of the temperature range, and that perhaps, the PCB is seeing this and making the pump run to try and dissipate hot water from around the heat exchanger until temp sensor value changes and it thinks things have cooled down. However, as the sensor is defective and not registering the drop in temperature, the pump runs indefinitely?
And on the face of it, I don't see any reason, notwithstanding that the DHW temp sensor IS clearly defective, why this would cause inconsistent DHW temperature when the gas valve doesn't appear to be modulating.
Would welcome the wisdom and experience of those more learned than myself...
Finally... In case it's relevant... I suspect that the expansion vessel may have ruptured... Pressure was 0.6 bar, but when I depressed the schrader valve, a fair amount of water was discharged... as pressure was within spec and boiler seems to be operating satisfactorily, I opted not to drain it completely as I didn't want to create any more issues and leave customer with no heat at all.
Can't, on the face of it, see that this would have any effect on DHW temp, but perhaps it's just something I'm missing.
Any responses very gratefully received as always.
